As semiconductor devices move toward higher integration and density with advances in semiconductor materials and processes, the critical defect size becomes smaller. This requires defect inspection tool, especially Electron Beam (e-beam) defect inspection tool has higher resolution and higher inspection speed. Thus, more accurate and fast auto-focus ability becomes more important.
A typical e-beam defect inspection tool projects electron beam onto the suspected area, collects the scattered or reflected electrons emanated form the surface of a specimen to form image of the inspected area, and then exports the position of the image anomalies. In order to have a good quality of surface image, the resolution of the inspection tool and the ability to keep good focus status. Higher resolution results in small depth of focus (DOF), thus require better and faster focus control capability.
After a sharp image is acquired from an wafer surface by using, for example e-beam inspection tool, anomaly analysis requires a lot of image processing techniques. There are several factors affect wafer surface topology for example vertical position and tilt angle. However, the most important three may be first, the wafer intrinsic flatness; second, the IC manufacturing induced surface tension and surface roughness; and third, the processing tool for example wafer chuck or x-y translation stage induced wafer warping and tilting. The surface topology is easy to cause the image off focus if no support of auto focus/work distance control system. Without precise knowledge of the vertical position and dynamic adjustment, the image will be blurred due to not enough DOF and surface topology variation. To present a sharp image while performing continuous inspection, the e-beam system has to adjust the magnetic field of the objective lens in time by changing the current flow through the coil of the objective lens. However, because of the hysteresis effect, the responding time of altering the focus by varying the lens current is too slow compared to the sample moving speed; thus the acquired image is obscure after image processing. Example of varying focus current is illustrated in U.S. Pat. No. 5,502,306 to Meisburger et al. The optimum focus current of a few designated points on specimen is determined in the initialization stage and for any points in between these one may interpolate the desired focus current. Another method to keep the image sharp was disclosed in U.S. Pat. No. 7,041,976 by Neil et al. The method automatically focuses an electron image by varying wafer surface bias which is correspondingly determined by the energy filter cut-off voltage of the scattered electrons.
Systems used to manufacture semiconductor devices such as processing tools, metrology tools, and inspection tools may include a height sensor as a focusing sub-system. A height sensor may be used to position a wafer within a system prior to or within the processing of the wafer. Height sensors may be used in different configurations for different applications.
Example of height sensor as illustrated in U.S. Pat. No. 4,538,069 to Shambroom et al. utilizes capacitance height sensor in lithography machine. The control system appropriately adjusts the deflection angle of the electron beam in response to the detected deviation to write at the desired point of the reticle with a very high degree of accuracy. However, the accurate reading of the capacitance gage requires a uniform specimen material. The complicate material compositions on a semiconductor wafer become a disadvantage of capacitance gage. Some height sensors detect the scattered light and convert the detected light into electrical signals that may be measured to provide height information. Examples of such height sensors are illustrated in U.S. Pat. No. 5,585,629 to Doran et al. Determine position deviation with dual beam optical LED. Output from the system enable adjustment of fine positioning stage; U.S. Pat. No. 6,333,510 to Watanabe et al. Determine height with dual beam optical system and apply the scale factor to adjust the focus of objective lens, and U.S. patent application Ser. No. 11/759,138 to Wang et al, Determine height with an optical system and adjust focal point by controlling Piezo-electric of the stage. All of which are incorporated by reference as if fully set forth herein. The disadvantage of implement height variation to adjust stage motion in vertical direction is that motions become a new vibration source of the imaging system. And the disadvantage of implement height variation to adjust objective lens exciting current is slow due to the hysteresis effect of the magnetic field in the objective lens.
The object of the present invention is to provide methods and apparatus to avoid the stage motion on the vertical direction and introduce a fast responded electrostatic field during dynamic focusing for wafer continuous e-beam defect inspection.